Pulmonary edema is a serious medical condition in which an excessive amount of fluid accumulates in the lungs. This condition usually results from heart failure. As such, some patients who have these heart conditions may also be prone to fluid accumulation in the lungs, that is, pulmonary edema. Heart failure patients may benefit from having an implantable cardiac rhythm management (CRM) device or an implantable resynchronization therapy device. Such implantable medical devices (IMDs) include, for example, implantable pacemakers, cardiac resynchronization therapy (CRT) devices, and implantable cardioverter-defibrillators, or “ICDs”, which may also have pacing functionality.
IMDs are usually implanted under the skin in the chest region, and they are designed to detect and treat irregular cardiac conditions via their attached implanted leads and electrodes. IMDs may sense ventricular tachycardia and/or ventricular fibrillation and deliver an electrical shock or other appropriate therapy in response to detecting such rhythms. They have one or more leads with one or more electrodes positioned in or around the heart to supply electrical stimulation to stabilize the heart rhythm, or to resynchronize ventricular contraction. In some IMDs, the housing of the IMD is conductive and serves as a large “can” electrode.
It is possible to use an IMD to detect changes in the volume of the fluid in the lungs by making an electrical impedance measurement across the lungs using the attached implanted electrodes. The more fluid there is in the lungs, the lower the impedance will be. U.S. patent application Ser. No. 10/303,305, filed on Nov. 25, 2002 by the present inventors, describes one way to measure lung impedance using an IMD to make an electrical impedance measurement between an electrode positioned epicardially over the left ventricular wall and connected to the IMD, and another electrode at the IMD itself. The impedance is measured by applying an electrical stimulus current to the implanted electrodes and measuring the resulting voltage using other implanted electrodes, and then calculating the ratio of voltage to current. This method measures the impedance of the thoracic tissues, including the lungs. The impedance is an indication of the fluid volume in the lungs. This measurement may be repeated over time to detect changes in impedance that indicate changes in lung fluid volume. It is also possible to detect fluid changes in the lungs by making an electrical impedance measurement using only external electrodes attached to the skin. In addition, it is possible to detect the volume of fluid in chambers of the heart by making an electrical impedance measurement using “pill” electrodes, swallowed into the esophagus, and external electrodes attached to the skin.